Heat exchanger



Jan. 9, 1940. Q w, G, NOACK 2,186,877

HEAT EXCHANGER Filed Feb. 3, 1959 32;, Frizz, P YW,

Patented Jan. 9 1940 UNITED STATES PATENT "OFFICE 2.136.877 nearEXCHANGER Application February 3, 1939, Serial No. 254,512

In Germany February I, 1938 I Claims. In recent years metal alloys havebeen made available which are capable of withstanding high temperatures.By the use of such alloys it is now possible to replace heat exchangers,such as 5 hot blast heaters made of refractory ceramic material andgenerally designed to operate intermittently, by metallic regeneratorsor recuperators designed to operate continuously. These alloys, however,do not permit the use of gases 10 at the highest temperatures attainableby the combustion of fuel with a slight excess of air.

The temperature of the hot combustion gases.

' sometimes is inconvenient and the use of a large excess of air in thecombustion is uneconomical because the large volume of waste gases carryaway heat. It is desirable therefore andis an object of the presentinvention to provide a system and a method of operating it whereby thetemperature of the gases entering the heat exchanger is lowered to thedesired extent without the necessity for-using a large excess of air inthe combustion of the fuel or generating steam or an otherwise utilizingthe gases at their highest temperature.

According to the present invention the hot gases, 'e. g., combustiongases. before entering the heat regenerator or recuperator are mixed :5with heating gas which has already passed through the regenerator andgiven up'part of its heat content. .The temperature of the hot gases isthus lowered to the desired extent without the heat loss incident toburning the fuel 40 with a large excess of air and without generatingsteam or cooling the combustion gases. in someother manner which may beinconvenient or uneconomical. This expedient solves the principm probleminvolved in the use of metals or 46 alloys in heat regenerators orrecuperators. An-

' other problem, however, is the high cost of the metal or alloy used inthe construction of the device. In order to reduce the size and cost ofthe recuperator one may resort to passing the 60 gases at high velocity.However, if the heatexchange surfaces were made as small as is possiblefor the necessary heat transfer when the combustion gases alone werepassed at high velocity, the passages in the recuperator would be sosmall ll that therecuperator would bediflcult to manufacture. If,however, the hot combustion gases are mixed with a portion of the gasesleaving the recuperator the volume of the mixture is sufficient, evenwhen passed at high velocity, towarrant a construction of therecuperator which is 5 easy to manufacture and still economical intheuse of the expensive metal or alloy.

The recuperator may be operated on the Velox principle, i. e., bypassing the gases through it not only at high velocity but also at highpres- 10 sure. Such operation calls for the service of a compressorwhich may be driven by a gas turbine which in turn may be driven by thehot gases themselves. In this method of operation, inorder to avoid toohigh temperatures in the re- 15 cuperator it may appear to be. desirableto employ a steam boiler of the Velox type in the path of the gases ontheir way to the recuperator in order ,to use the heat of the gases attheir highest temperature for generating steam. Accord- 2g ing to thepresent invention, however, the use of a steam generator for loweringthetemperature of the gases on their way to the regenerator orrecuperator is avoided by the return of a portion of the gases leavingthe recuperator to the 25 stream of gases on their way to therecuperator.

Specifically these partially cooled gases may be withdrawn from thestream of gases passing from the recuperator to the gas turbine. At thispoint the gases are at such a temperature that 3 they may be handledwithout cooling and their recompression to the pressure of the gasesentering the recuperator requires only the small amount of increase inpressure corresponding to thefiow resistance of the recuperator.

'The invention will be more fully understood by. reference to theaccompanying drawing in which Fig. 1 is a diagrammatic showing of asource of combustion gases, a recuperator and means for returning aportion of the gases leav- 40 big the recuperator to the gases enteringthe same and Fig. 2 is a diagrammaticshowing of a system designed forburning the fuel .under pressure.

' In Fig. l, lis a combustion chamber, 2 is the as supply of fuel andair to the combustion chamber, 3 is the heat exchanger, for instance ahot blast heater designed as a recuperatonl and 5 are the inlet andoutlet respectively of the combustion gases into and out of the heat ex-50 changer, 6 and I are the inlet and outlet respectively. of the gas tobe heated by heat exchange withthe hot combustion gases, 8 is a motorand ii a fan driven bymotor 8 for returning aportion of the gasesleaving the regenerator 3 tp the combustion chamber I. The movement thegases is indicated by arrows. As appears, he stream of gases leaving theregenerator 3 divides and a portion only of it is returned to thecombustion chamber for admixture with the hot combustion gases passingto the regenerator while the remainder is disicrhaziged to waste or toother means for utiliz- In Fig. 2 the combustion chamber I is operatedon the Velox principle with both the air and fuel supplies underpressure. Fuel and air are supplied under pressure by the blowers orpumps l0 and II. The hot. combustion gases from the combustion chamberpass into the second stage heat exchanger l2 as indicated at 4 and passout at 5. Here the stream is divided, part of it passing to the fan orblower 9 which delivers it to the combustion chamber for admixture withthe hot products of combustion thereby producing a gas mixture having asufficiently lowered temperature to handle in the heat exchanger. Theother portion of the gasesleaving the second stage heat exchanger passesto the gas turbine l 3 and finally through the first stage of the heatexchanger l4. Turbine I3, driven by the waste gases, drives the fans 9,l0 and II. I5 is an auxiliary motor, e. g., an electric motor which maysupplement the turbine 13. The gases to be heated enter the first stage.heat exchanger 14 at 6 and then pass to the second stage heat exchangerI2 and are delivere therefrom at 1.

The temperature of the gas delivered to the heat exchanger may be variedat will without loss of emciency. When the amount of gas in circulationis increased, its speed is increased and the heat transfer is therebyincreased, the heat transfer surface remaining constant. The increasedheat transfer compensates for the lower temperature of the gas passingthrough the re- 7 generator.

The great advantage of the method of operation described is that itleaves the combustion chamber operation entirely independent of thetemperature limitations of the metallic heat exchanger and permits thecombustion of the fuel at maximum efliciency. The gases returned to thecombustion chamber from the regenerator may even serve to cool the wallsof the combustion chamber and thus lengthen its life.

The invention is of course applicable to any hot gases from which heatis to be recovered and with respect to whicha lowering of theirtemperature and an increase in their volume is desirable.

1. Apparatus for hat' exchange comprising a source of hot heatinggasesunder high pressure, a metallic heat exchanger, means fordelivering the hot heating gases from the source thereof 3. Apparatus asdefined in claim 1 comprising a combustion chamber designed to beoperated under pressure, compressors for supplying fuel and air to saidcombustion chamber under pressure, a turbine connected to drive saidcompressors, a gas pump driven by said turbine connected to delivergases into the hot heating gases pro duced in the combustion chamber,means for delivering a portionof the heating gases leaving the heatexchanger to said gas pump and means for delivering the remainder of.said gases to drive the gas turbine.

4. Apparatus for heat exchange comprising a combustion chamber designedto operate under pressure, compressors for delivering fuel and air tosaid chamber under pressure, a second stage recuperator having heattransfer walls made. of metal, means for passing hot heating gases fromthe combustion chamber through said second stage recuperator, a gaspump, means for delivering a portion of the heating gases leaving saidsecond stage recuperator to said gas pump, means for deliveringsaidportion of the heating gases high pressure combustion chamber, aheat exchanger having metallic. heat transfer walls, means fordelivering hot heating gases from said combustion chamber to said heatexchanger and means for delivering a portion of theheating gases afterpassage through said heat exchanger into contact with the walls of saidcombustion chamber and then into admixturewith the hot heating gasesleaving said combustion chamber.

WALTER GUSTAV NOACK.

